In the 10 update, the Prey System was implemented. All players who participated in the fight against bosses receive an individual changes of the creature (get extra damage, experience or others..).

From each of the prey slots you can choose one creature out of 9 creatures. These 9 creatures are randomly selected from a large pool of creatures, but there will always be some low-level creatures, some mid-level creatures and some high-level creatures.

The moment you select a creature you receive a bonus with it. The bonuses are:

(player) damage boost of 7% - 25% (steps of 2%);

(creature) damage reduction of 12% - 30% (steps of 2%);

bonus experience 13% - 40% of (steps of 3%);

improved loot 13% - 40% of (steps of 3%).

1. If you don't like the bonus you received you can do one of the following:

Buy a Prey Bonus Reroll in the Store with Tibia Coins;

Hunt the creature with the bonus anyways for 2 hours and hope for better luck next time.
2. Each bonus has 10 steps and a reroll will always improve your current step (up to the maximum) and may change your bonus type. If you are already at step 10 the bonus type is guaranteed to change with a reroll.
3. The improved loot bonus works in the following manner. If you have an improved loot bonus with an effect of 15% this means there is a 15% chance that the monster will have another set of loot generated, basically it will behave as if you killed the monster twice.

Note: the information about this in the Prey System you can check on tibia.wiki external website.